The present invention relates to a magnetic type pickup cartridge, and more particularly a moving-magnet type stereophonic pickup cartridge which is extremely simple in construction and may exhibit considerably improved desired characteristics such as frequency response.
In general, a moving-magnet type pickup cartridge consists of a moving unit consisting of a stylus bar with a stylus attached to one end thereof and with a moving magnet attached to the other end thereof and supported in a sleeve made of a non-magnetic material through a damper made of an elastic material, and a voltage generating or inducing unit consisting of a yoke made of a high-permeability material and coupled to the moving unit in such a way that the variation in magnetic flux flowing through the yoke may be induced by the movement of the moving unit and consequently a voltage may be induced across a coil mounted on the yoke. The quality of the pickup cartridges is, in general, dependent upon how faithfully the variation in magnetic flux caused in the moving unit is transmitted to the voltage generating unit. Therefore, the loss in the magnetic circuit must be minimized, and the frequency response characteristic must be as flat as possible over the audible range. In the conventional pickup cartridge, a balanced winding of two coils for each channel is employed in order to cancel externally induced hum, so that four coils are employed in the stereophonic pickup cartridge. In addition the complete separation between the right and left channels must be ensured, and the magnetic circuits for the right and left channels must be symmetrical with the same length of the magnetic path.
Even in the conventional moving-magnet type stereophonic pickup cartridges designed to satisfy the above and other requirements as much as possible, the length of the magnetic circuit or path is different between the right and left channels, as will be described in detail hereinafter, so that the balanced magnetic characteristics cannot be attained. In addition, the pole pieces and yokes each fabricated by drawing of a permalloy sheet are used, because ferrite is not adapted to be drawn, so that various magnetic losses occur. As a result, the flat frequency response characteristic cannot be attained, and the response at high frequencies is considerably attenuated. The reason is mainly attributed to the eddy-current loss caused by the skin effect due to the dimensions and configurations of the pole pieces and yokes made of permalloy. In order to overcome these drawbacks it has been proposed to use laminated pole pieces and yokes, but the sensitivity drops. Furthermore, the fabrication of the laminated pole pieces and yokes is very complex because the thin sheets formed by the drawing must be subjected to heat-treatment in order to improve the magnetic properties and because the thin sheets must be laminated without being subjected to any mechanical distortions.